1. Field of the Invention
This invention relates to quality assurance methods used for quality assurance for laser shock peening and, more particularly, for ultrasonic testing and statistical analysis of laser shock peened surfaces for quality assurance of a production laser shock peening process.
2. Discussion of the Background Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled xe2x80x9cAltering Material Propertiesxe2x80x9d; U.S. Pat. No. 4,401,477, entitled xe2x80x9cLaser Shock Processingxe2x80x9d; and U.S. Pat. No. 5,131,957, entitled xe2x80x9cMaterial Propertiesxe2x80x9d. Laser shock peening, as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force by instantaneous ablation or vaporization of a painted or coated or uncoated surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No.4,937,421, entitled xe2x80x9cLaser Peening System and Methodxe2x80x9d. These methods typically employ a curtain of water flowed over the workpiece or some other method to provide a confining medium to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP""D to create the beneficial compressive residual stresses.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. Nos.: 5,756,965 entitled xe2x80x9cON THE FLY LASER SHOCK PEENINGxe2x80x9d; U.S. Pat. No. 5,591,009, entitled xe2x80x9cLaser shock peened gas turbine engine fan blade edgesxe2x80x9d; U.S. Pat. No. 5,569,018, entitled xe2x80x9cTechnique to prevent or divert cracksxe2x80x9d; U.S. Pat. No. 5,531,570, entitled xe2x80x9cDistortion control for laser shock peened gas turbine engine compressor blade edgesxe2x80x9d; U.S. Pat. No. 5,492,447, entitled xe2x80x9cLaser shock peened rotor components for turbomachineryxe2x80x9d; U.S. Pat. No. 5,674,329, entitled xe2x80x9cAdhesive tape covered laser shock peeningxe2x80x9d; and U.S. Pat. No. 5,674,328, entitled xe2x80x9cDry tape covered laser shock peeningxe2x80x9d, all of which are assigned to the present Assignee. These applications, as well as others, are in need of efficient quality assurance testing during production runs using laser shock peening.
LSP is a deep treatment of the material and it is desirable to have a quality assurance test that is indicative of a volumetric LSP effect. It is also desirable to have a QA method that is compatible with a dual sided or simultaneous dual sided LSP process wherein substantially equal compressive residual stresses are imparted to both sides of a workpiece, i.e. along the leading edge of a gas turbine engine fan blade.
One laser shock peening quality assurance technique previously used is high cycle fatigue (HCF) testing of blades having leading edges which are LSP""d and notched in the LSP""d area before testing. This method is destructive of the test piece, fairly expensive and time consuming to carry out, and significantly slows production and the process of qualifying LSP""d components. An improved quality assurance method of measurement and control of LSP that is a non-destructive evaluation (NDE), inexpensive, accurate, and quick is highly desirable. It is also desirable to have an NDE quality assurance method that is relatively inexpensive and sufficiently economical to be used on each workpiece instead of a sampling of workpieces. LSP is a process that, as any production technique, involves machinery and is time consuming and expensive. Therefore, any techniques that can reduce the amount or complexity of production machinery and/or production time are highly desirable.
Interferometric profilometry method and apparatus to obtain volumetric data of a single laser shock peened test dimple created with a single firing of a laser used in the laser shock peening process is disclosed in U.S. Pat. No. 5,948,293 xe2x80x9cLaser shock peening quality assurance by volumetric analysis of laser shock peened dimplexe2x80x9d. Other QA methods are disclosed in U.S. Pat. No. 5,987,991 xe2x80x9cDetermination of Rayleigh wave critical anglexe2x80x9d; U.S. Pat. No. 5,974,889 xe2x80x9cUltrasonic multi-transducer rotatable scanning apparatus and method of use thereofxe2x80x9d; and U.S. Pat. No. 5,951,790 xe2x80x9cMethod of monitoring and controlling laser shock peening using an in plane deflection test couponxe2x80x9d.
A method for quality control testing of a laser shock peening process of a production workpiece includes (a) ultrasonically scanning at least a portion of a laser shock peened surface on the workpiece wherein a region having deep compressive residual stresses imparted by the laser shock peening process extends into the workpiece from the laser shock peened surface, (b) digitizing a signal derived from the scanning and forming a digitized image of intensity values from the scanning, (c) calculating at least one statistical function value for a plurality of points of the digitized image of the workpiece based on the intensity values, and (d) comparing the statistical function value to a pass or fail criteria for quality assurance of the laser shock peening process or accepting or rejecting the workpiece.
In one exemplary embodiment of the invention the plurality of points of the digitized image (44) are delineated by a group of circles corresponding to laser shock peened dimples within the portion of the laser shock peened surface. The statistical function comprises at least one of four statistical properties of the digitized image defined by four equations, a Mean Matrix MM(k) for each kth dimple, a Dimple Standard Deviation Matrix SDM(k), a Mean Vector MV(x) of all the points in the group of circles, where x is the number of pixels in each dimple, and a Standard Deviation Vector SDV(x) of each of the group of circles. Three types of the statistical function are a Mean of Dimple Mean Matrix F1, a Mean of Standard Deviation Matrix F2, and a Mean of Standard Deviation Vector F3.
In another exemplary embodiment of the invention the plurality of points of the digitized image are delineated by a rectangle around laser shock peened dimples within a portion of the laser shock peened surface and the statistical function is a Sobel Function F4 including a Sobel operator.
The pass or fail criteria is based on a pre-determined correlation of test piece statistical function data and high cycle fatigue failure based on high cycle fatigue tests of test pieces that were laser shock peened in the same or similar laser shock peenin apparatus. Each of the test pieces has a failure precipitating flaw within a laser shock peened area of the test piece that was laser shock peened in the same or similar laser shock peening apparatus.